The present invention relates to a quantum well structure composed of quantum well layers and barrier layers of an energy gap larger than that of the quantum well layers and a semiconductor device employing such a quantum well structure.
On account of the progress of thin film growth techniques in recent years, it is now possible to obtain, with ease and excellent controllability, what is called a multiple quantum well structure wherein thin films of two materials of different energy gaps are alternately laminated to thicknesses substantially equal to the de Broglie's wavelength of electrons. The multiple quantum well confines electrons or holes in the thin films through utilization of the difference in energy gap between the two materials, and hence produces a variety of quantum effects unobtainable with bulk crystals, and the effects are now being widely applied to devices.
It has been made clear, theoretically and experimentally, that a semiconductor laser using the multiple quantum well structure, for example, has low oscillation threshold current density and is less temperature-dependent than a conventional double hereto laser and suitable for a narrow spectrum operation, a high-speed operation and a high output operation, and hence has excellent characteristics.
It has also been ascertained that selective doping of an impurity into only the barrier layer in the multiple quantum well structure suppresses scattering of carriers confined in the quantum well layer by impurity atoms and thus would contribute to further enhancement of device characteristics. For example, an HEMT is a device which utilizes such selective doping, and in this instance, carriers from the barrier layer doped with an n-type impurity are stored in a non-impurity-doped active layer to provide high electron mobility. Furthermore, it has been reported that the application of the selectively-doped multiple quantum well structure would increase a differential gain by p-type impurity doping and decrease a threshold current by n-type impurity doping (Uomi et al., Skingaku Giho, OQD86-66, for example).
The present invention is provided to solve the problems of the prior art which will hereinbelow be described in respect of drawings. FIG. 7 is a schematic diagram of an AlGaAs semiconductor laser which is a typical example of conventional selective-doping multiple quantum well semiconductor lasers. On an n-type GaAs substrate 1 there are laminated an n-type AlGaAs layer 2 serving as an n-type clad layer, selectively doped multiple quantum well layers 11 and 12 serving as active layers, a p-type AlGaAs layer 3 serving as a p-type clad layer and a p-type GaAs cap layer 4 for contact with an electrode, and electrodes 101 and 102 for current injection use are formed on the top and underside of the assembly, respectively. For laterial mode stability and current contraction a semi-insulating AlGaAs layer 5 is used to form a buried stripe geometry. FIGS. 8A and 8B show the band diagrams and impurity concentration in the vicinity of the active layers of the laser. Four p-type AlGaAs barrier layers 12 and five non-doped quantum well layers 11 are alternately laminated into the active layers. Each barrier layer 12 is doped with Be which is a p-type impurity (i.e. an acceptor), with a uniform concentration (1.times.10.sup.18 cm.sup.-3). The p-type impurity ions (Be-) bend the energy bands of the barrier layers and the quantum well layers 11. Electrons 201 injected via the electrode 102 are injected from the n-type AlGaAs layer 2 into the quantum well layers 11, wherein they are recombined with holes 202, thereby emitting photons. At this time, since the electrons 201 are injected into the quantum well layers 11 according to their potential difference, the farther the electrons 201 go away from the n-type AlGaAs clad layer 2, the more barrier layers 12 they go over and the higher energy they have. The energy distribution of electrons is based on the Fermi distribution function, and since the distribution decreases with an increase in the energy of electrons, the number of electrons 201 injected into the respective quantum well layer 11 decreases with an increase in the distance of the quantum well layer from the n-type AlGaAs clad layer 2. 0n the other hand, the holes 202 which are emitted by the ionization of Be doped in the barrier layers 12 are localized in the quantum well layers, and for the same reason as given above in the case of the injected electrons, the distribution of holes which are injected via the electrode 101 into the quantum well layers 11 from the p-type AlGaAs clad layer 3 has such a tendency that the number of holes injected into the respective quantum well layer decreases with an increase in the distance from the p-type AlGaAs clad layer 3 (i.e. a decrease in the distance from the n-type AlGaAs clad layer 2). Thus, the distributions of the electrons 201 and the holes 202 are opposite in tendency to each other.
For the reasons given above, this structure has shortcomings that when the number of quantum well layers is large, it is difficult to distribute carriers injected from the outside uniformly throughout the entire multiple quantum well structure, and that the light emitting efficiency is low because the distributions of injected electrons and holes have opposite tendencies.